1. Field of the Invention
The present invention relates to an image heating apparatus (heating apparatus of an electromagnetic induction heating system) which is used as a fixing apparatus for heating/fixing hot melting type powder images (visualizing agent images) such as toner on a recording material (recording sheet) in image forming apparatuses such as an electrophotographic copying machine and a printer.
2. Related Background Art
In conventional image forming apparatuses such as an electrophotographic copying machine, a heat roller system apparatus is for general use as an image heating/fixing apparatus.
FIGS. 7A, 7B are schematic views showing one example of the heat roller system image heating/fixing apparatus. The apparatus is provided with a pair of rotary rollers comprising a fixing roller (heat roller) 100 as a heating rotary member with heat sources such as a halogen heater (halogen lamp) H1 incorporated therein and a pressurizing roller 200 pressed in contact with the fixing roller to form a heating nip portion (fixing nip portion) N. In the apparatus, a recording material P bearing a formed and unfixed toner image t as the material to be heated is introduced to the heating nip portion N which is a roller pair pressed portion and pinched and conveyed so that the unfixed toner image t is heated/pressurized on the surface of the recording material P and fixed (heated and melted) as a permanent fixed image.
The surface temperature of the fixing roller 100 as the heat roller needs to be controlled and accurately maintained at a predetermined fixing temperature so that the temperature exceeds the melting point of toner t and the recording material P is prevented from being adversely affected.
Therefore, the temperature adjusting method by ON-OFF control shown in the example of a control circuit of FIG. 7C has been heretofore used in many cases. Specifically, when an alternating-current voltage is supplied between input terminals a and b, the alternating-current voltage is applied to a solid state relay SSR via a heater H1 and an operable state is obtained.
Here, the temperature control circuit (temperature adjusting circuit) starts a temperature control, this temperature (control circuit reads surface temperature information detected temperature) from a temperature detecting element (temperature measuring element) TH1 such as a thermistor for measuring the surface temperature of the fixing roller 100, compares the information with a temperature control target value, determines a proportional heater energizing time in accordance with a difference, and turns on the solid state relay SSR to start supplying power to the halogen heater or the like as the heat source H1.
Thereafter, when the surface temperature of the fixing roller 100 as the heat roller approaches the control target value, the ratio during the heater energizing is determined in accordance with the difference between the target value and the detected temperature from the temperature detecting element TH1, and the solid state relay SSR is turned on/off to stabilize the surface temperature of the fixing roller 100.
In the constitution in which the halogen heater is used as the heat source H1 to warm the fixing roller 100 as the heat roller by radiant heat, since a heater current needs to be supplied at a certain constant time interval, the surface temperature of the fixing roller 100 disadvantageously fluctuates with a certain width. Moreover, since ON-OFF is repeated at the constant time interval, an excessive rush (inrush) current flows during re-lighting the constant time after the halogen heater is turned off, and a power source flicker trouble as a recent social problem is easily caused.
Therefore, the present inventors et al. have conducted researches on an electromagnetic induction heating system heating apparatus in which a rotary member itself generates heat and heat efficiency is enhanced so that the rotary member forms no heat resistance.
The present applicant has proposed such type of apparatus in U.S. Pat. No. 5,568,240.
A magnetic field generated by magnetic field generating means, for example, by combining a core material as a magnetic member with an energizing coil is changed in an energizing circuit. Specifically, an eddy current is generated in a conductive layer of a rotary member by applying a high frequency to the coil and repeatedly generating and extinguishing the magnetic field in the rotary member as a conductive member (induction magnetic material, magnetic field absorbing conductive material) which moves in the generated magnetic field. This eddy current is converted to heat (Joule heat) by the electric resistance of the conductive layer, and as a result the rotary member adhering to the recording material generates heat. Therefore, this heating apparatus is superior in heat efficiency.
However, for the above-described conventional electromagnetic induction heating system heating apparatus, since in the nip portion, a heat radiation amount is larger in an end than in a center, the amount of heat applied to the recording material cannot be uniformed, thereby causing a problem that heating insufficiency and fixing failure are caused in the end and the toner is offset to the rotary member in the center.
Therefore, the present applicant proposes in U.S. Pat. No. 5,552,582 that the temperature distribution in a nip longitudinal direction is uniformed by adjusting a core material, adjusting a core height, or adjusting the area of the core occupying the nip.
However, the adjustment of the core material and core height tends to relatively complicate the apparatus constitution. Moreover, the adjustment of the core area which occupies the nip tends to waste the core, and further improvements have been demanded.